The present invention relates to a gray cast iron plate alloy for brake disks, and more particularly to the use of such an alloy for brake disks of utility vehicles.
Disk brakes which are partially acted upon and made of gray cast iron are generally customary nowadays at least on the front wheels of passenger vehicles. The gray cast iron alloys used for this purpose are optimized with respect to comfort aspects, such as freedom from squealing and rubbing, because these aspect are dominant in the passenger car realm. The life expectancies of the brake disks are easily met by the alloys used in this field of application.
In utility vehicles, however, particularly with long-distance trucks or construction site vehicles, which are much heavier than passenger cars, the life expectancies are much higher than in the passenger car realm. In this case, a life in the range of several hundreds of thousands of kilometers is expected of a brake drum before a required replacement. As a rule, the service brakes of the utility vehicles are therefore constructed as drum brakes which have a much lower specific stress level in comparison to disk brakes. Inasmuch as disk brakes offer various advantages in comparison to drum brakes, disk brakes have recently been introduced in utility vehicle construction. If the alloy of the utility vehicle brake drums which also consist of gray cast iron were to be used in order to cast from it brake disks for utility vehicle disk brakes, the life expectancies existing for brake disks could not be met due to the higher stress levels.
In addition, because of the much higher thermal stressing of brake disks in comparison with brake drums, so-called "heat cracks" would also occur relatively early if the drum alloy were used for brake disks. Heat cracks are cracks which extend radially in the brake area and are initiated by repetitive high temperature stress and subsequent cooling and at first, that is, in small number and/or sizes of the cracks, are harmless. As the stress continues, the heat cracks increase in number, and existing heat cracks connect with others and expand radially. Thus, the heat cracks will, at some point, reach the exterior disk edge. If the brake disk were not at the latest repaired then, there would be the danger of a surface break out of the brake area or even complete disk breakage.
It is an object of the present invention to provide a gray cast iron plate alloy for brake disks which is optimized with respect to a higher service life and a reduced heat crack formation.
This object has been achieved according to the present invention by providing an alloy for a gray cast iron plate for casting blanks of brake disks, having the composition--in percent by weight--of
3.65 to 3.95%, preferably approximately 3.85% carbon, PA1 0.20 to 0.40%, preferably approximately 0.30% chromium, PA1 1.80 to 2.20% silicon, PA1 0.60 to 0.80% manganese, PA1 0.30 to 0.50% copper, PA1 0.20 to 0.80%, preferably approximately 0.38 to 0.45% niobium,
rests of iron, traces of other alloy metals and melting-caused impurities, such as phosphorus or sulfur.
The gray cast iron alloy according to the present invention is a high-carbon hypereutectic iron base material. Without the other alloy constituents, particularly niobium, a soft basic structure having an insufficient stability and hardness would occur due to the high carbon and silicon contents. The high carbon and silicon content provides, however, a good thermal conductivity of the casting material which, in turn, results in a reduction of the formation of heat cracks.
It should be noted that the relatively high carbon content is, among other things, paired with a specific moderate niobium content. As a result, despite the high carbon and silicon content, a complete and mainly finely lamellar, pearlitic basic structure is obtained. The latter is important for the high tensile strength and hardness.
According to the wall thickness of the brake disk (in the case of lower wall thicknesses, less; in the case of larger wall thicknesses, more), the silicon content is to be adjusted such that a Brinell hardness of approximately 150 to 190 HB is obtained and that the tensile strength is in the range of from 15 to 18 N/mm.sup.2. In the fine-pearlitic basic structure, the niobium carbides are embedded in a uniformly distributed manner and reduce the wear during the braking operation. Also, a sufficiently good machinability is obtained of the cast alloy with the fine-pearlitic basic structure. The alloy constituents or contents of the present invention, in the combination described herein, provide a higher tensile strength, an increased resistance to wear and an also increased resistance to heat cracking than conventional brake disk alloys in gray cast iron plates.
It is important that, for use as a brake disk, the graphite carbon embedded in the cast structure is deposited in plate form because the plate form of graphite is much more advantageous for the tribological stressing of the brake disk than a ball shaped graphite. Thus, a nodular gray cast iron would not be suitable for a brake disk, also because of the increased danger of heat cracking, although this type of gray cast iron has a higher tensile strength and, at least in the bainitized condition, also a higher resistance to wear compared with the plate form gray cast iron. Alloys made of nodular gray cast iron were not previously known for the purpose of solving present usage and service life problems. In particular, prior art directed to passenger car brake disk alloys or drum brake alloys did not make this recognition.